Not all harmful impurities in potable waters are detectable by taste and/or odor. Nitrates, for example, are undetectable by the senses yet them may be physiologically harmful. In humans, especially in infants, consumed nitrates may be reduced to nitrites in the gastrointestinal tract. Upon absorption into the bloodsteam, nitrites react with hemoglobin to produce methemoglobin, which impairs oxygen transport.
The presence of nitrate in municipal water supplies is becoming an urgent problem in some locations at its level increases due to the use of nitrogen fertilizers and to pollution.
Investigators of nitrate ion exchange phenomena are familiar with the interference other ions exert on nitrate removal by anion resins. All major anions found in groundwater consume resin capacity although not to the same degree. This reduces nitrate removal efficiency of the resin and increases the quantity of regenerant chemicals. It is understandable that various investigators have sought some ideal "nitrate-selective resin" as a remedy for the observed process inefficiencies and complexities. Such a resin could remove nitrate ions only and allow other anions to pass; thus preserving some original qualities of the untreated water. At the end of the run, the resin would be nitrate loaded making the regeneration of resin more efficient. However, in practice with available resins, interfering ions cannot only alter the quality of product water but also present the possibility of producing a higher nitrate level if the nitrate breakthrough is exceeded. This latter effect is called "reverse adsorption" or "autoregeneration" and can occur if sulfate ion is present. The product water differs from treated water by having bicarbonate, nitrate, and sulfate partially or completely replaced by chloride. In some cases, this can result in a water exceeding secondary chloride standards and a water low in bicarbonate and high in pH and corrosivity. The higher the affinity of the resin for nitrate compared to other ions, the fewer product water quality problems will be encountered.
The presence of significant amounts of dissolved sulfate ion (about 50 ppm or more), in particular, has been an impediment to nitrate removal. A review of work on nitrate removal by strongbase ion exchangers is given by Gauntlet. (Gauntlet, R. B., "Nitrate Removal From Water By Ion Exchange, Water Treatment and Examination", Water Treatment and Examination, Vol. 24, Part 3 (1975), pp. 172f.) The early work reported by Gauntlet shows that irrespective of the resin used sulfate ions were adsorbed more strongly by the resins. Using various resins and waters of varying nitrate and sulfate composition in column tests, it was demonstrated that resin capacity for nitrate decreased with increasing feedwater sulfate content. Gauntlet compared the chemical efficiency of complete column regeneration with partial regeneration and found much better efficiency for partial regeneration for high sulfate waters and moderate improvement for low sulfate waters. He believed that complete regeneration of a single bed to have the disadvantage of producing a corrosive high chloride to alkalinity ratioed product.
An ion exchange plant for nitrate removal was put into operation in 1969 in Garden City Park, N.Y. (Sheinker, M., and J. Codoluto, "Making Water Supply Nitrate Removal Practicable", Public Works, June, 1977, pp. 71f.) The plant uses a strong-base anion exchange resin in a continuous flow loop system (Higgins, I. R., "Continuious Ion Exchange Equipment", Ind. and Eng. Chem., 53, 1961, p. 336.) The influent water has a 14.9 mg/nitrate nitrogen content. No sulfates are reported but are believed to be quite low.
Midkiff, W. S., and W. J. Weber, "Operating Characteristics of Strong Base Anion Exchange Reactors", Engineering Bulletin, Prudue University Extension Service, 1970, 37, 593-604, reported significant work on nitrate removal with DOWEX 21K strong-base anion resin. Column operation was examined for a water containing both nitrate and sulfate.
Korngold, E., "Removal of Nitrates from Potable Water by Ion Exchange", Wat. Air Soil Pollution, 1973, 2, 15-22, reports experiments with Amberlite-400 and high chloride, low sulfate nitrate laden waters. His results show typical breakthrough curves for the four major anions. Of major interest is his brief study on the use of seawater as a regenerant.
Buelow, R. W. et al, "Nitrate Removal by Anion-Exchange Resins", Journal AWWA, September 1975, pp. 528f, investigated a reported nitrate selective resin and how specific anions interfere with anion resins in nitrate removal service. The DOWEX 21K which was reported as "nitrate selective" by Chemical Separations Corporation did not show nitrate selectivity in waters of TDS (total dissolved solids) concentrations of about 500 ppm. At high TDS levels, the resin did show nitrate selectivity. This was pointed out to be expected because of the monovalent ion preference in feedwaters of high ionic concentrations. Sulfate, iron, and silica were pointed out to be the most problematical.
Dalton, G. L., "The Removal by Ion Exchange of Nitrates From Borehole Water at Aroab SWA", report of the National Institute for Water Research, Pretoria, 1978, studied the application of nitrate removal by ion exchange to waters in the Republic of South Africa and southwest Africa. Amberlite IRA 904 resin was selected from 32 resins as having the highest nitrate-to-sulfate selectivity and was used in pilot plant tests. The waters tested were of high TDS allowing electroselectivity effects to give high nitrate-to-sulfate selectivity.
Grinstead, R. R., and K. C. Jones, "Nitrate Removal from Wastewaters by Ion Exchange", EPA report 17010FSJ01/71, January 1971, reported studies on porous polymer beads carrying alkyl substituted amidines. Typical K.sub.Cl.sup.N values ranged from 15 through 40 and log K.sub.S.sup.N values were 5 to 6 (NSS=1.69 to 2.69). Their object was to increase K.sub.Cl.sup.N of an ion exchanger for removal of nitrate from wastewaters of high chloride content and thereby obtain better efficiency. Loss of extractant from the polymer beads to the product stream and the low ion exchange capacity of the resin of below 0.2 eq/l were the observed disadvantages of the process. The high K.sub.Cl.sup.N of the resin precluded the possibility of obtaining high nitrate concentrations in the waste regenerant.
U.S. Pat. No. 4,134,861 issued to Roubinek (Diamond Shamrock) adopted the amidine approach of Grinstead and Jones. The patent describes a method of resin preparation by introduction of the amidine groups into a preformed polymer which is preferably a vinyl polymeric matrix cross-linked by divinylbenzene, resulting in the recurring structure --CH.sub.2 --CHX--, wherein X is the amidine group. It was found that the best balance between nitrate selectivity and ease of regeneration is obtained when the total number of carbon atoms on the amidine was 5 to 7. Butyl and ethyl groups were generally preferred. K.sub.Cl.sup.N values were reported to be 7 to 10. No K.sub.S.sup.N values were reported nor were any column tests or regeneration efficiencies reported.
Clifford, D. A., and W. J. Weber, "Nitrate Removal From Water Supplies by Ion Exchange", EPA-600/2-78-052, June 1978, reported an exhaustive study comparing the ionic selectivities of 19 strong-base and 13 weak-base commercial resins. They found that for groundwaters having TDS concentrations up to at least 3,000 ppm all resins preferred sulfate to nitrate. Clifford et al found that nitrate-to-sulfate selectivity among strong-base resins increased to some degree, dependent on the degree of cross-linking, but was unaffected by the number of carbon atoms surrounding the ammonium nitrogen atom. Clifford et al found the reverse to be true in the case of weak-base resins, i.e., that nitrate-to-sulfate selectivity increases with increasing R group size but that the degree of cross-linking in the substrate resin has no significant effect. The Clifford et al report also concluded (at page 6) that "Sulfate/nitrate selectivity was nearly irrelevant in determining the average equivalent fraction of nitrate on the resin at the end of a run" and that higher sulfate selectivity increases rather than decreases the amount of nitrate at the end of a column run. Thus, the findings of Clifford et al seem to lead to the conclusion that increases in nitrate/sulfate selectivity would not improve the column performance of resins for nitrate removal in the presence of sulfate. In any event, the commercial resins studied by Clifford et al did not show enough nitrate selectivity to be of significant value and should not be regarded as nitrate-to-sulfate selective (NSS) resins.
In summary, a need for anion exchange resin having significantly higher nitrate-to-sulfate selectivity remains a long-standing need in the art. If higher nitrate selectivity could be translated into a higher capacity for adsorbed nitrate, a NSS resin might offer significant regenerant savings because, in general, the more nitrate on a resin at the end of the run, the more nitrate will be removed per pound of regenerant. Whether or not a given resin is fully loaded with nitrate, 4 to 6 bed volumes of a 6 percent saline solution are required to effect regeneration. However, any increase in capacity which might be afforded by higher nitrate-to-sulfate selectivity might be partially or wholly offset by an increase in nitrate-to-chloride selectivity which would render the resin more difficult to regenerate. Such an offset was seen in the work of Walitt and Jones who incorporated nitrate analytical reagents into polystyrene to make an anion exchange resin selective for nitrate. They concluded "It appears that we have chosen to examine in depth a specific example of the proposed concept which was far too successful; that is, the affinity of the 1-naphthylmethylamine group (in the polystyrene resin) for nitrate ion is so great that regeneration to the free base by ordinary methods is unsuccessful." Walitt, A. L., and H. L. Jones "Basic Salinogen Ion-Exchange Resins for Selective Nitrate Removal from Potable and Effluent Waters", U.S. EPA, Cincinnati, Advanced Waste Treatment Laboratory, 1970, U.S. GPO, Washington, D.C. Thus, "nitrate selective" is terminology which can imply both advantages and disadvantages.